Question about global environment variables and fork() & exec() [duplicate]

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This question already has an answer here:



  • what does it mean 'fork()' will copy address space of original process

    1 answer



What I'm trying to understand is where does a child process inherit exported variables from it's parent process



What I'm trying to understand is where a child process inherits exported environment variables from? I understand the exec() system call overwrites environment variable created by the fork()... But how does exec() cause exported environment variables to be included with a newly created child process if all the environment variables are overwritten? My best guess is that exported environment variables are somehow excluded from being overwritten... But I can't confirm this



EDIT I edited my question to be clearer with what I am asking










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marked as duplicate by roaima, Scott, Mr Shunz, GAD3R, muru Dec 19 at 14:00


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.














  • You say “But how does exec() …. newly created child process”. However exec never creates a new process.
    – ctrl-alt-delor
    Dec 18 at 20:27










  • A related question with the same underlying misconception is unix.stackexchange.com/questions/473001 .
    – JdeBP
    Dec 19 at 7:57















0















This question already has an answer here:



  • what does it mean 'fork()' will copy address space of original process

    1 answer



What I'm trying to understand is where does a child process inherit exported variables from it's parent process



What I'm trying to understand is where a child process inherits exported environment variables from? I understand the exec() system call overwrites environment variable created by the fork()... But how does exec() cause exported environment variables to be included with a newly created child process if all the environment variables are overwritten? My best guess is that exported environment variables are somehow excluded from being overwritten... But I can't confirm this



EDIT I edited my question to be clearer with what I am asking










share|improve this question















marked as duplicate by roaima, Scott, Mr Shunz, GAD3R, muru Dec 19 at 14:00


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.














  • You say “But how does exec() …. newly created child process”. However exec never creates a new process.
    – ctrl-alt-delor
    Dec 18 at 20:27










  • A related question with the same underlying misconception is unix.stackexchange.com/questions/473001 .
    – JdeBP
    Dec 19 at 7:57













0












0








0








This question already has an answer here:



  • what does it mean 'fork()' will copy address space of original process

    1 answer



What I'm trying to understand is where does a child process inherit exported variables from it's parent process



What I'm trying to understand is where a child process inherits exported environment variables from? I understand the exec() system call overwrites environment variable created by the fork()... But how does exec() cause exported environment variables to be included with a newly created child process if all the environment variables are overwritten? My best guess is that exported environment variables are somehow excluded from being overwritten... But I can't confirm this



EDIT I edited my question to be clearer with what I am asking










share|improve this question
















This question already has an answer here:



  • what does it mean 'fork()' will copy address space of original process

    1 answer



What I'm trying to understand is where does a child process inherit exported variables from it's parent process



What I'm trying to understand is where a child process inherits exported environment variables from? I understand the exec() system call overwrites environment variable created by the fork()... But how does exec() cause exported environment variables to be included with a newly created child process if all the environment variables are overwritten? My best guess is that exported environment variables are somehow excluded from being overwritten... But I can't confirm this



EDIT I edited my question to be clearer with what I am asking





This question already has an answer here:



  • what does it mean 'fork()' will copy address space of original process

    1 answer







process environment-variables exec fork






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share|improve this question













share|improve this question




share|improve this question








edited Dec 19 at 16:58

























asked Dec 18 at 19:10









Bodisha

232




232




marked as duplicate by roaima, Scott, Mr Shunz, GAD3R, muru Dec 19 at 14:00


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.






marked as duplicate by roaima, Scott, Mr Shunz, GAD3R, muru Dec 19 at 14:00


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.













  • You say “But how does exec() …. newly created child process”. However exec never creates a new process.
    – ctrl-alt-delor
    Dec 18 at 20:27










  • A related question with the same underlying misconception is unix.stackexchange.com/questions/473001 .
    – JdeBP
    Dec 19 at 7:57
















  • You say “But how does exec() …. newly created child process”. However exec never creates a new process.
    – ctrl-alt-delor
    Dec 18 at 20:27










  • A related question with the same underlying misconception is unix.stackexchange.com/questions/473001 .
    – JdeBP
    Dec 19 at 7:57















You say “But how does exec() …. newly created child process”. However exec never creates a new process.
– ctrl-alt-delor
Dec 18 at 20:27




You say “But how does exec() …. newly created child process”. However exec never creates a new process.
– ctrl-alt-delor
Dec 18 at 20:27












A related question with the same underlying misconception is unix.stackexchange.com/questions/473001 .
– JdeBP
Dec 19 at 7:57




A related question with the same underlying misconception is unix.stackexchange.com/questions/473001 .
– JdeBP
Dec 19 at 7:57










2 Answers
2






active

oldest

votes


















3














There are no global environment variables. They are passed from parent to child.




  • fork does not change the environment variables.


  • exec without the e post-fix does not change the environment variables.


  • exec with e post-fix overrides environment variables.

As well as using the e post-fixed execs to change the environment, you can also do:



int pid = fork() //new process with same environment variables
if (pid == 0)
putenv //add some environment variables
unsetenv //remove some environment variables
exec //replace program. non e version of exec.






share|improve this answer




























    2















    int execve(const char *filename, char *const argv,
    char *const envp);


    -- man execve




    The envp parameter provides all the environment variables of the executed program. If you pass no values in envp, the executed program will see no environment variables at all.



    When you read an environment variable (getenv()), you always read it from the current process.



    There is no way to set a system-wide environment variable. You can only configure the environment of your initial processes, setting variables to be passed on to child processes.



    So it is possible to start another program with a completely different environment. But the most widely useful and common convention, is to start programs with a copy of your own environment. You might modify specific environment variables if you need to.



    One exception is su --login and sudo --login ..., which cleanly reset the environment. Omitting the --login option to these commands allows some environment variables to be passed through... this sometimes causes "unexpected results".






    share|improve this answer






















    • When I said "global environment variables" I meant exported variables. If Bash has exported variables & a user starts a program. How & when does a child process acquire the exported variables from the parent process? You seemed to confirm how I thought this was happening when you said "start programs with a copy of your own environment". So does the exec() overwrite the region of memory the fork() created... But the child process inherits exported variables because they are NOT overwritten... Can you confirm if my understanding is accurate or not? Thanks!
      – Bodisha
      Dec 19 at 16:53











    • @Bodisha look at the execve() system call again. The argv parameter specifies the arguments to pass to the new program, as a list of strings (char *const ... ). The envp parameter specifies the environment variables, as a list of strings. So you can see environment working very similarly to arguments. (The strings should be formatted as NAME=value). If you also want to know a specific implementation detail, you should ask a specific question, specifying exactly which implementation you are asking about :-P. And mention why you want to know.
      – sourcejedi
      Dec 19 at 18:06











    • @Bodisha the set of exported shell variables (as shown by export -p) is the set of environment variables. Non-exported shell variables are not environment variables. This Wikipedia section is good: en.wikipedia.org/wiki/Environment_variables#Unix
      – sourcejedi
      Dec 19 at 21:05

















    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    3














    There are no global environment variables. They are passed from parent to child.




    • fork does not change the environment variables.


    • exec without the e post-fix does not change the environment variables.


    • exec with e post-fix overrides environment variables.

    As well as using the e post-fixed execs to change the environment, you can also do:



    int pid = fork() //new process with same environment variables
    if (pid == 0)
    putenv //add some environment variables
    unsetenv //remove some environment variables
    exec //replace program. non e version of exec.






    share|improve this answer

























      3














      There are no global environment variables. They are passed from parent to child.




      • fork does not change the environment variables.


      • exec without the e post-fix does not change the environment variables.


      • exec with e post-fix overrides environment variables.

      As well as using the e post-fixed execs to change the environment, you can also do:



      int pid = fork() //new process with same environment variables
      if (pid == 0)
      putenv //add some environment variables
      unsetenv //remove some environment variables
      exec //replace program. non e version of exec.






      share|improve this answer























        3












        3








        3






        There are no global environment variables. They are passed from parent to child.




        • fork does not change the environment variables.


        • exec without the e post-fix does not change the environment variables.


        • exec with e post-fix overrides environment variables.

        As well as using the e post-fixed execs to change the environment, you can also do:



        int pid = fork() //new process with same environment variables
        if (pid == 0)
        putenv //add some environment variables
        unsetenv //remove some environment variables
        exec //replace program. non e version of exec.






        share|improve this answer












        There are no global environment variables. They are passed from parent to child.




        • fork does not change the environment variables.


        • exec without the e post-fix does not change the environment variables.


        • exec with e post-fix overrides environment variables.

        As well as using the e post-fixed execs to change the environment, you can also do:



        int pid = fork() //new process with same environment variables
        if (pid == 0)
        putenv //add some environment variables
        unsetenv //remove some environment variables
        exec //replace program. non e version of exec.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Dec 18 at 20:50









        ctrl-alt-delor

        10.8k41957




        10.8k41957























            2















            int execve(const char *filename, char *const argv,
            char *const envp);


            -- man execve




            The envp parameter provides all the environment variables of the executed program. If you pass no values in envp, the executed program will see no environment variables at all.



            When you read an environment variable (getenv()), you always read it from the current process.



            There is no way to set a system-wide environment variable. You can only configure the environment of your initial processes, setting variables to be passed on to child processes.



            So it is possible to start another program with a completely different environment. But the most widely useful and common convention, is to start programs with a copy of your own environment. You might modify specific environment variables if you need to.



            One exception is su --login and sudo --login ..., which cleanly reset the environment. Omitting the --login option to these commands allows some environment variables to be passed through... this sometimes causes "unexpected results".






            share|improve this answer






















            • When I said "global environment variables" I meant exported variables. If Bash has exported variables & a user starts a program. How & when does a child process acquire the exported variables from the parent process? You seemed to confirm how I thought this was happening when you said "start programs with a copy of your own environment". So does the exec() overwrite the region of memory the fork() created... But the child process inherits exported variables because they are NOT overwritten... Can you confirm if my understanding is accurate or not? Thanks!
              – Bodisha
              Dec 19 at 16:53











            • @Bodisha look at the execve() system call again. The argv parameter specifies the arguments to pass to the new program, as a list of strings (char *const ... ). The envp parameter specifies the environment variables, as a list of strings. So you can see environment working very similarly to arguments. (The strings should be formatted as NAME=value). If you also want to know a specific implementation detail, you should ask a specific question, specifying exactly which implementation you are asking about :-P. And mention why you want to know.
              – sourcejedi
              Dec 19 at 18:06











            • @Bodisha the set of exported shell variables (as shown by export -p) is the set of environment variables. Non-exported shell variables are not environment variables. This Wikipedia section is good: en.wikipedia.org/wiki/Environment_variables#Unix
              – sourcejedi
              Dec 19 at 21:05















            2















            int execve(const char *filename, char *const argv,
            char *const envp);


            -- man execve




            The envp parameter provides all the environment variables of the executed program. If you pass no values in envp, the executed program will see no environment variables at all.



            When you read an environment variable (getenv()), you always read it from the current process.



            There is no way to set a system-wide environment variable. You can only configure the environment of your initial processes, setting variables to be passed on to child processes.



            So it is possible to start another program with a completely different environment. But the most widely useful and common convention, is to start programs with a copy of your own environment. You might modify specific environment variables if you need to.



            One exception is su --login and sudo --login ..., which cleanly reset the environment. Omitting the --login option to these commands allows some environment variables to be passed through... this sometimes causes "unexpected results".






            share|improve this answer






















            • When I said "global environment variables" I meant exported variables. If Bash has exported variables & a user starts a program. How & when does a child process acquire the exported variables from the parent process? You seemed to confirm how I thought this was happening when you said "start programs with a copy of your own environment". So does the exec() overwrite the region of memory the fork() created... But the child process inherits exported variables because they are NOT overwritten... Can you confirm if my understanding is accurate or not? Thanks!
              – Bodisha
              Dec 19 at 16:53











            • @Bodisha look at the execve() system call again. The argv parameter specifies the arguments to pass to the new program, as a list of strings (char *const ... ). The envp parameter specifies the environment variables, as a list of strings. So you can see environment working very similarly to arguments. (The strings should be formatted as NAME=value). If you also want to know a specific implementation detail, you should ask a specific question, specifying exactly which implementation you are asking about :-P. And mention why you want to know.
              – sourcejedi
              Dec 19 at 18:06











            • @Bodisha the set of exported shell variables (as shown by export -p) is the set of environment variables. Non-exported shell variables are not environment variables. This Wikipedia section is good: en.wikipedia.org/wiki/Environment_variables#Unix
              – sourcejedi
              Dec 19 at 21:05













            2












            2








            2







            int execve(const char *filename, char *const argv,
            char *const envp);


            -- man execve




            The envp parameter provides all the environment variables of the executed program. If you pass no values in envp, the executed program will see no environment variables at all.



            When you read an environment variable (getenv()), you always read it from the current process.



            There is no way to set a system-wide environment variable. You can only configure the environment of your initial processes, setting variables to be passed on to child processes.



            So it is possible to start another program with a completely different environment. But the most widely useful and common convention, is to start programs with a copy of your own environment. You might modify specific environment variables if you need to.



            One exception is su --login and sudo --login ..., which cleanly reset the environment. Omitting the --login option to these commands allows some environment variables to be passed through... this sometimes causes "unexpected results".






            share|improve this answer















            int execve(const char *filename, char *const argv,
            char *const envp);


            -- man execve




            The envp parameter provides all the environment variables of the executed program. If you pass no values in envp, the executed program will see no environment variables at all.



            When you read an environment variable (getenv()), you always read it from the current process.



            There is no way to set a system-wide environment variable. You can only configure the environment of your initial processes, setting variables to be passed on to child processes.



            So it is possible to start another program with a completely different environment. But the most widely useful and common convention, is to start programs with a copy of your own environment. You might modify specific environment variables if you need to.



            One exception is su --login and sudo --login ..., which cleanly reset the environment. Omitting the --login option to these commands allows some environment variables to be passed through... this sometimes causes "unexpected results".







            share|improve this answer














            share|improve this answer



            share|improve this answer








            edited Dec 18 at 20:51

























            answered Dec 18 at 20:24









            sourcejedi

            22.8k436100




            22.8k436100











            • When I said "global environment variables" I meant exported variables. If Bash has exported variables & a user starts a program. How & when does a child process acquire the exported variables from the parent process? You seemed to confirm how I thought this was happening when you said "start programs with a copy of your own environment". So does the exec() overwrite the region of memory the fork() created... But the child process inherits exported variables because they are NOT overwritten... Can you confirm if my understanding is accurate or not? Thanks!
              – Bodisha
              Dec 19 at 16:53











            • @Bodisha look at the execve() system call again. The argv parameter specifies the arguments to pass to the new program, as a list of strings (char *const ... ). The envp parameter specifies the environment variables, as a list of strings. So you can see environment working very similarly to arguments. (The strings should be formatted as NAME=value). If you also want to know a specific implementation detail, you should ask a specific question, specifying exactly which implementation you are asking about :-P. And mention why you want to know.
              – sourcejedi
              Dec 19 at 18:06











            • @Bodisha the set of exported shell variables (as shown by export -p) is the set of environment variables. Non-exported shell variables are not environment variables. This Wikipedia section is good: en.wikipedia.org/wiki/Environment_variables#Unix
              – sourcejedi
              Dec 19 at 21:05
















            • When I said "global environment variables" I meant exported variables. If Bash has exported variables & a user starts a program. How & when does a child process acquire the exported variables from the parent process? You seemed to confirm how I thought this was happening when you said "start programs with a copy of your own environment". So does the exec() overwrite the region of memory the fork() created... But the child process inherits exported variables because they are NOT overwritten... Can you confirm if my understanding is accurate or not? Thanks!
              – Bodisha
              Dec 19 at 16:53











            • @Bodisha look at the execve() system call again. The argv parameter specifies the arguments to pass to the new program, as a list of strings (char *const ... ). The envp parameter specifies the environment variables, as a list of strings. So you can see environment working very similarly to arguments. (The strings should be formatted as NAME=value). If you also want to know a specific implementation detail, you should ask a specific question, specifying exactly which implementation you are asking about :-P. And mention why you want to know.
              – sourcejedi
              Dec 19 at 18:06











            • @Bodisha the set of exported shell variables (as shown by export -p) is the set of environment variables. Non-exported shell variables are not environment variables. This Wikipedia section is good: en.wikipedia.org/wiki/Environment_variables#Unix
              – sourcejedi
              Dec 19 at 21:05















            When I said "global environment variables" I meant exported variables. If Bash has exported variables & a user starts a program. How & when does a child process acquire the exported variables from the parent process? You seemed to confirm how I thought this was happening when you said "start programs with a copy of your own environment". So does the exec() overwrite the region of memory the fork() created... But the child process inherits exported variables because they are NOT overwritten... Can you confirm if my understanding is accurate or not? Thanks!
            – Bodisha
            Dec 19 at 16:53





            When I said "global environment variables" I meant exported variables. If Bash has exported variables & a user starts a program. How & when does a child process acquire the exported variables from the parent process? You seemed to confirm how I thought this was happening when you said "start programs with a copy of your own environment". So does the exec() overwrite the region of memory the fork() created... But the child process inherits exported variables because they are NOT overwritten... Can you confirm if my understanding is accurate or not? Thanks!
            – Bodisha
            Dec 19 at 16:53













            @Bodisha look at the execve() system call again. The argv parameter specifies the arguments to pass to the new program, as a list of strings (char *const ... ). The envp parameter specifies the environment variables, as a list of strings. So you can see environment working very similarly to arguments. (The strings should be formatted as NAME=value). If you also want to know a specific implementation detail, you should ask a specific question, specifying exactly which implementation you are asking about :-P. And mention why you want to know.
            – sourcejedi
            Dec 19 at 18:06





            @Bodisha look at the execve() system call again. The argv parameter specifies the arguments to pass to the new program, as a list of strings (char *const ... ). The envp parameter specifies the environment variables, as a list of strings. So you can see environment working very similarly to arguments. (The strings should be formatted as NAME=value). If you also want to know a specific implementation detail, you should ask a specific question, specifying exactly which implementation you are asking about :-P. And mention why you want to know.
            – sourcejedi
            Dec 19 at 18:06













            @Bodisha the set of exported shell variables (as shown by export -p) is the set of environment variables. Non-exported shell variables are not environment variables. This Wikipedia section is good: en.wikipedia.org/wiki/Environment_variables#Unix
            – sourcejedi
            Dec 19 at 21:05




            @Bodisha the set of exported shell variables (as shown by export -p) is the set of environment variables. Non-exported shell variables are not environment variables. This Wikipedia section is good: en.wikipedia.org/wiki/Environment_variables#Unix
            – sourcejedi
            Dec 19 at 21:05


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